1. Field of the Invention
The present invention relates generally to remedial well bore sealing methods and compositions, and more particularly, to improved methods and compositions for sealing subterranean zones whereby their permeabilities are reduced and their strengths are increased.
2. Description of the Prior Art
In the drilling of oil and gas wells using the rotary drilling method, drilling fluid is circulated through the drill string and drill bit and back to the surface by way of the well bore being drilled. The drilling fluid maintains hydrostatic pressure on the subterranean zones through which the well bore is drilled and circulates cuttings out of the well bore. The drilling fluid includes gelled and particulate material which builds up on the walls of the well bore and prevents fluid loss into subterranean zones penetrated by the well bore. However, relatively large openings such as channels, fractures, vugs and the like are often encountered which communicate the well bore with one or more high permeable subterranean zones. Because of the size of such openings filter cake from the drilling fluid does not seal the openings and drilling fluid is lost from the well bore into the subterranean zones. This in turn causes drilling fluid circulation to be lost whereby the drilling operations must be terminated while remedial steps are taken to seal the subterranean zones. Highly permeable subterranean zones which produce water or other undesirable fluids into the well bore are also often encountered which must be sealed.
Heretofore, a variety of methods and sealing compositions have been developed and used for combating drilling fluid lost circulation problems and problems relating to formation fluids flowing into the well bore. However, such methods and compositions have often been unsuccessful due to inadequate viscosity development by the sealing compositions used. Also, the methods of placement of the sealing compositions have been inadequate to cause plugging of the weak highly permeable portions of the zones being treated and to prevent bypassing of such portions by the sealing compositions and/or the wash-out of the compositions.
After a well bore penetrating a subterranean hydrocarbon producing formation has been drilled, the well bore is often completed by sealing a string of pipe such as casing or a liner in the well bore. That is, a sealing composition such as a hydraulic cement slurry is pumped into the annular space between the walls of the well bore and the exterior of the string of pipe disposed therein. The cement slurry is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the well bore and bonds the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
The cement compositions utilized in cementing pipe strings in well bores must often be lightweight to prevent excessive hydrostatic pressures from being exerted on weak permeable zones penetrated by the well bores. In some applications, the heretofore utilized lightweight cement compositions have still had densities such that the cement compositions can not be displaced into well annuluses all the way to the surface due to the hydrostatic pressure of the cement compositions exceeding the fracture gradient of one or more of the weak permeable zones penetrated by the well bores. The resulting upper unsupported portion of the casing can and often does experience early damage due to formation cave-ins, subsidence and the like.
Thus, there is a continuing need for improved methods and sealing compositions for sealing highly permeable subterranean zones through which fluids undesirably flow into or out of the well bores penetrating the zones and for simultaneously increasing the mechanical strengths of the zones.